1. Field of the Invention
The present invention relates to passive infrared motion detectors, occupancy sensors and similar devices, and more particularly to the infrared input section and detection system of these devices.
2. Description of the Related Art
Passive infrared motion detectors and occupancy sensors employ an array of Fresnel lenses covering an entrance aperture. This lens array is illuminated by thermal infrared radiation from the object of interest. For any particular angle of incidence each of the elements in the lens array covering the entrance aperture generates a focal spot. The array of lenses is designed so that as the object of interest moves across its field of view the system of focal spots moves across the sensitive area of a detector. The varying electrical output signal generated by the detector is processed to yield information about the state of motion of the object of interest.
Each element of the lens array is designed to focus incident infrared radiation in a small angular range onto the sensitive area of a detector. The (active) angular sectors in which the elements of the lens array focus incident infrared radiation onto the sensitive area of a detector are interlaced by (passive) angular sectors within which incident infrared radiation is not focused onto the sensitive area of the detector by any element of the lens array. Moving infrared radiators are detected when they move from one angular sector across a boundary into an adjacent angular sector leading to a rapid change in the amount of infrared power falling on the sensitive area of the detector. Ordinarily, the sectors are all approximately the same angular size and the maximum angle through which a radiating object of interest can move without being detected, i.e. the angular resolution of the system, is equal to the angular size of one of these sectors. This assumes that the size and velocity of the radiating object and its distance from the entrance aperture are such that the intensity of the infrared signal is greater than the minimum that can be detected by the system electronics.
One way to improve the angular resolution of the system is to increase the number of elements in the lens array. More specifically, for an entrance aperture of fixed size the angular resolution of the system is approximately inversely proportional to the number of elements in the lens array. Thus, in order to achieve the smallest angular resolution, a lens array with as many elements as possible must be employed. On the other hand, the sensitivity and effective range of the system decrease if the size of the individual lens elements of the array is decreased. The phrase xe2x80x9csensitivity of the systemxe2x80x9d refers to the size of the smallest radiating object that can be detected as a function of its distance from the detector. Thus, compromises must be made between the size of the entrance aperture, sensitivity, range and angular resolution of the system. For example, for any desired sensitivity and range there is a minimum size for each of the individual lenses of the array and hence a maximum number of elements for an entrance aperture of fixed size and a corresponding minimum angular resolution.
The present invention is a new input lens configuration and detection scheme which can be employed, for example, to: 1) increase the sensitivity and range of motion detectors and occupancy sensors with an entrance aperture of fixed size without decreasing the angular resolution of the system or, 2) improve the angular resolution of a system with an entrance aperture of fixed size without decreasing the sensitivity or range of the system or, 3) decrease the size of the entrance aperture required for a given sensitivity, range and angular resolution or, 4) reduce the distance that the unit must protrude in, for example, a wallbox installation in order to achieve acceptable performance at wide angles. The relative importance of each of these characteristics of motion detectors and occupancy sensors depends on the application in which the system is employed.
In simplest terms, the infrared input section and detection scheme disclosed herein consists of an array of lens elements followed by a first radiation detector including a pair of sensitive areas and a second radiation detector having a pair of sensitive areas. The pair of dual detectors, such as, for example pyrodetectors are positioned so that as the direction of incident radiation varies, at least one element of the lens array at least partially focuses the incident radiation onto at least three of the four sensitive areas of the two detectors, thereby forming an effective quad-detector. Typical pyrodetectors used in prior art occupancy sensors and motion detectors have two sensitive areas on their surface each of which is 1 mm wide, separated by 1 mm. As a focal spot of any particular element of the lens array moves across this 3 mm region, four electrical signals are generated as the spot moves onto or off of a sensitive area. These signals are generated as the infrared source moves across the angular boundaries which separate regions of space where the focal spot is on a sensitive area as opposed to regions of space where the focal spot is not on one of the sensitive areas of the detector. Motion detectors and occupancy sensors which employ a detector with two sensitive areas can achieve improved performance as compared to units that employ a detector with only one sensitive area, since each focal spot gives rise to four electrical signals as the infrared source moves as opposed to two electrical signals for a detector with only one sensitive area. In many applications a detector system with three or four sensitive areas can improve the performance of the occupancy sensor/motion detector beyond that achievable with a detector with only two sensitive areas.
The difficulty with using two detectors side, by side in order to obtain four sensitive areas, is that because of the physical size of a pyrodetector the sensitive areas on one detector are much more than 1 mm from the sensitive areas on the other detector. Because of this, the angular regions which define those regions of space wherein a focal spot is on an sensitive area as opposed to those angular regions where a focal spot is not on a sensitive area are not even approximately uniformly distributed. Nevertheless the infrared input section and detection scheme disclosed herein is capable of producing an almost uniform distribution of these angular sectors. For the purpose of illustration, suppose that a certain range, angular resolution and sensitivity can be achieved by employing some particular lens array. If the number of elements of the array and their size is not changed, but a quad-detector of the type disclosed herein is employed, the number of electrical signals is effectively doubled as an infrared source moves across the field of view of the lens array. The angular resolution has been improved by a factor of two. Alternatively, if a quad-detector of the type disclosed herein is employed and the number of elements in the lens array is halved, the angular resolution is unchanged but the infrared power in the focal spot is doubled with a corresponding increase in range and sensitivity. In some applications the optimum design is a hybrid system which employs a traditional array of Fresnel lenses and/or mirrors to cover some angular ranges and a design of the type disclosed herein for other angular ranges.
Also, by employing a quad-detector of the type disclosed herein it is possible to achieve the same performance with a much smaller entrance aperture. This is of importance, for example, in applications where accidental damage or casual vandalism of the entrance aperture lens/cover is a problem. One aesthetically appealing configuration is a rocker switch (e.g. Leviton""s Decora rocker switch) with a small infrared entrance aperture in the center, both vertically and horizontally, of the rocker. This would convert the traditional rocker switch to an xe2x80x9cautomatic switchxe2x80x9d i.e. an ordinary switch with an occupancy sensor feature. This aesthetically appealing configuration can also be achieved without a quad-detector. However, a quad-detector can be employed to enlarge the field of view and/or decrease the required aperture size for a given sensitivity and range. This technique can be applied to other wiring devices e.g. toggle switches, dimmers, timers, outlets, etc. These new designs maintain the traditional appearance of the device while adding the occupancy sensor/motion detector feature in an inconspicuous way. As previously noted in each of these applications a quad-detector of the type disclosed herein may or may not be employed depending on the specified size of the entrance aperture and the required field of view, sensitivity and range.
For any occupancy sensor or motion detector, the field of view can be increased by employing mirrors adjacent to the entrance aperture to reflect wide angle rays towards the center of the system. These mirrors may be positioned before or after the lens array. Further, in some applications the optimum system is a hybrid in which the mirrors direct and/or focus infrared radiation from some angular sectors directly onto a detector while infrared radiation in other angular sectors is passed through the lens array to the detector. Infrared radiation from other angular sectors may be processed differently.
All of the preceding is equally applicable to, for example, wall and ceiling units, indoor and outdoor units in lighting, heating, ventilation and/or security applications. Also, it is equally applicable to passive and active infrared and optical systems. Further, the implementations disclosed herein may be used in single technology systems or in combination with motion detectors/occupancy sensors based on other technologies e.g. active infrared, ultrasonic or microwave systems.